Comparative study of physico-chemical, acid–base and catalytic properties of vanadium based catalysts in the oxidehydrogenation of n-butane: effect of the oxide carrier (original) (raw)
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Butane dehydrogenation on vanadium supported catalysts under oxygen free atmosphere
Applied Catalysis A: General, 2004
The present study investigates the catalytic n-butane dehydrogenation under oxygen free atmosphere using VO x supported on USY, NaY, ␥-Al 2 O 3 and ␣-Al 2 O 3 . Experiments are developed at 520 • C in a fixed bed microreactor. Catalyst characterization via TPR demonstrates the presence of different VO x species with the nature of these species being a function of the catalyst support. The acidic properties of the prepared catalysts are also evaluated using NH 3 TPD with the following order of acidity being observed: VO X /␥-Al 2 O 3 > VO x M/␣-Al 2 O 3 > VO x /USY. Reactivity experiments show that the VO x /USY provides both the highest catalytic activity and butane selectivity, with the VO x /␥-Al 2 O 3 displaying a lower activity and butane selectivity and the other tested catalyst samples being essentially inactive. The superior performance of the VO x /USY catalyst is assigned to its mild acidity.
n-Butane selective oxidation on vanadium-based oxides : Dependence on catalyst microstructure
1986
The oxidation of n-butane and of its intermediates to maleic anhydride has been studied on different vanadium-phosphorus oxides and on Ti02-and zeolites-supported vanadium oxides. On vanadium-phosphorus oxides the activity in n-butane selective oxidation depends strongly on the catalyst microstructure. On supported vanadium oxides n-butane is not selectively oxidized ; however, when the amount of vanadium deposed largely exceeds the monolayer amount, low yields of acetic acid are obtained. The analysis of the oxidation of some intermediates suggests that the mechanism of maleic anhydride formation from n-butane occurs through the successive formation of butadiene, 2,5-dihydrofuran and furan via successive cycles of oxygen insertion and allylic H-abstraction and that these properties are connected to the vanadium ions and not to a particular surface structure. On the contrary, the alkane activation requires a particular surface structure of vanadium deriving by straining of V-(OP) connections. A model of the possible mechanism for n-butane activation iS also given. INTRODUCTION Notwistanding the growing interest in the selective oxidation of n-butane to maleic anhydride (11, the nature of the active sites able to activate the paraffins is not clear and the only hypothesis made in the literature, involving the Presence Of D-species (21, is lacking the experimental support necessary in order to extraPol_ ate the results to real catalysts. Furthermore, due to the very complex and.multisteps reaction pattern, lacking knowledges are present in literature on the nature and structure of the active sites necessary for the successive steps from n-butane to maleic anhydride. Aim of this work was to analyze our data of the selective oxidation of n-butane and of its possible intermediates to maleic anhydride on different vanadium-based catalysts, for the Purpose of determining the reaction mechanism and the nature and ?roPerties of the active centers for the successive steps from n-butane to maleic 0166-9834/86/$03.50 0 1986 Elsevier Science Publishers B.V. anhydride. EXPERIMENTAL Vanadium-phosphorus oxides (VP): V205 was reduced in 37 % HCl, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting solution was concentrated and then water added to obtain a blue precipitate which was dried at 150 C for 24 h (VP a*). ~205 was reduced in a mixture (3:2) of isobuthyllbenzyl alcohols, then o-H3P04 added to obtain a P:V atomic ratio of 1.0. The resulting slurry was filtered and dried at 150 C for 24 h (VP b*). Both precursor samples VP(a*) and VP(b*) were then activated in a flow of 1% n-butane/air at 400 C for 6 h, to give the VP(a) and VP(b) catalysts, respectively. Vanadium-supported oxides : VT1 18 and VTi 117 catalysts were prepared by impregnation with a NH4V03/oxalic acid/water solution, drying at 150 C for 24 h and calcination at 430 C for 3 h. Both Ti02 supports in the anatase form were Tioxide, CLDD 1587/ /2 (18.4 m2/g) and CLDD 1764/Z (117 m*/g) respectively. The amount of vanadium deposited in both cases was 10% wt of V205. Zeolites-supported vanadium oxides (VZ) were prepared by impregnation with a NH4V03 solution of Y-zeolite [ VZ(HY) 1, HZSM-5 or HZSM-11 [ VZ(ZSM5) and VZ(ZSM11) j and by ionic exchange with VOS04 solution of Y-zeolite [ V02+ Y ]. Amount of deposed vanadium is about 2 % wt of V205 and 3.5% wt in the case of V02+Y . Before catalytic tests, samples were activated in air at 430 C for 6 h. Further details on the preparation of all these catalysts and on their characterization have been reported previously (j-10). Catalytic tests Catalytic tests were performed in a flow reactor with analysis on-line of the reagent composition and products of reaction by means of two gas-chromatographs. Details of the reactor and method of analysis are reported elsewhere (8). One g of catalyst was used for each test. The reagent composition was hydrocarbon:oxygen:nitrogen 0.6:12.0:87. The total flow of the reactant was 70 cc/min. RESULTS Vanadium-phosphorus oxides The catalytic behaviors of VP(a) and VP(b) catalysts in n-butane and 1-butene selective oxidation are reported in Figure 1. Catalyst VP(b) is more active than catalyst VP(a) both in alkene and alkane oxidation, in agreement with the higher surface area (27 and 6 m2/g, respectively). However the difference in activity is much greater in the n-butane than in 1-butene oxidation. Furthermore, whereas the V -+(0-P) bond, the presence of medium-strong Lewis acidity due to Va atoms can be thus explained in the catalyst VP(a) surface. However the enhanced Lewis acidity of the catalyst VP(b) surface cannot be explained. The (020) planes are connected by pyrophosphate groups and thus disorder in the stacking-fold of the (020) planes induces the straining of the V~lr~~(O-Pl bonds, which can be schematically represented as follows : A SCHEME 2 The Lewis acidity of the Vb atoms is enhanced by this effect as compared to the Lewis acidity of the corresponding Va atoms. It has been shown on solid super acid (16) that the first step in the activation of n-butane is the extraction of an H-from the n-butane by very-strong Lewis sites. Similarly, it is possible to hypothesize that the very-strong Lewis sites observed on catalyst VP(b) and to a lesser extent on catalyst VP(a), are the sites responsible for the first step in alkane activation on vanadium-phosphorus oxides. It is thus possible to propose the mechanism of n-butane selective activation showed in Scheme 3. A coordinated attack of itrong Lewis sites ( Vb atoms ) and of a strong base ( OS-) activates the n-butane, giving the corresponding olefins which are further quickly oxidized due to their higher reactivity. Relationship between structure and mechanism of oxidation --In a previous work we have showed (9) that the general mechanism of maleic anhydride formation from n-butane can be written as follows : n-butane -Dbutenes --Obutadiene -4furan -+maleic anhydride 7 G.Centi, Z.Tvaruzkova, F.TrifirB, P.Jiru and L.Kubelkova, Appl. Catal., 13 (19841 69. 8 G.Centi, G.Fornasari and F.Trifir6, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985) 32. 9 G.Centi, G.Fornasari and F.Trifird, J. Catal., 89 (1984) 44. 10 F.Cavani, G.Centi and F.Trifir6, Appl. Catal., 15 (1985) 151. 11 E.Bordes, P.Courtine and J.W.Johnson, J. Solid State Chem., 55 (1984) 270. 12 J.
Applied Catalysis A: General, 2004
In the present work, many different vanadium based catalysts have been prepared and tested in the oxidative dehydrogenation (ODH) of butane. The catalysts were prepared both by impregnation and grafting for a useful comparison. In particular, the grafting procedure was studied by contacting solutions of increasing concentrations of vanadyl tri-isopropoxide, dissolved in dioxane, with two different supports: SiO 2 and TiO 2 -SiO 2 . Grafting adsorption behaviour was studied using the isotherms obtained in the two cases. On silica we obtained an isotherm with two different recognisable plateaux. On the contrary, in the adsorption on TiO 2 -SiO 2 the system followed the Langmuir adsorption law with a saturation value at about 7% of adsorbed V 2 O 5 . By greatly increasing the vanadyl concentration multi-layer adsorption seems to occur. In this latter case, catalysts obtained after calcination resulted less dispersed. Each point of both the mentioned isotherms corresponds to a catalyst with more or less dispersed vanadium. All these catalysts were characterised by using many different techniques (X-ray diffraction (XRD), diffuse reflectance spectroscopic analysis (DRIFT and DRUV) and laser-Raman spectroscopy (LRS) and XPS) and tested in the ODH of butane. In particular, we observed that vanadium dispersed catalysts are more active and selective. From the observation of the adsorption isotherms it is possible to decide the best conditions for preparing well dispersed catalysts with a relatively high vanadium load that have higher activities and selectivities.
Journal of Catalysis, 1997
The oxidation of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO 2 , ZrO 2 , CeO 2 , Nb 2 O 5 , Al 2 O 3 , and SiO 2 ). No correlation was found between the properties of the terminal V= =O bond and the butane oxidation turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidation TOF nor maleic anhydride selectivity was related to the extent of reduction of the surface vanadia species. The n-butane oxidation TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Brønsted acidity and decrease in the number of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidation TOF by ca. 50 (Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addition of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidation TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an especially positive effect on the maleic anhydride selectivity.
Oxidative dehydrogenation of propane and n-butane over aluminia supported vanadium catalysts
Latin American …, 2004
Structural properties of vanadium dispersed species on γ γ γ γ-A1 2 0 3 are investigated with the scope to detect changes related with V loading in the oxidative dehydrogenation (ODH) of propane and nbutane. XPS, FTIR, and FTIR of absorbed pyridine were used to study the nature of vanadium supported species. Tetrahedral V 5+ and probably V 4+ species were detected. For vanadium loadings higher than 4.3 % wt octahedral species were also observed. In the n-butane ODH reaction, the selectivity to ODH products decreases when vanadium content increases. However, for propane ODH, the selectivity seems to be independent of vanadium loadings. Low oxygen/alkane feeding ratios favor selectivity to olefins. It is also shown that low V loading catalysts reach selectivities as good as best reported V-Mg-O catalyst.
Ethane Oxydehydrogenation over Supported Vanadium Oxides
Industrial & Engineering Chemistry Research, 1994
Ethane oxydehydrogenation to produce ethylene was studied over vanadium oxides supported on either silica gel, HZSM-5, silicalite, or A1P04-5. Characterization of the catalysts was carried out by X-ray photoelectron spectroscopy for the vanadium binding energy, temperature-programmed desorption of ammonia for the number of acidic sites on the supports, and redox titration for the number of chemisorption sites on vanadium oxides. The results of kinetic studies showed that the ethane oxydehydrogenation rates over vanadium oxide were enhanced by the presence of acidic sites on the supports. The rate equations thus derived suggested that the reduction of vanadium oxide sites by adsorbed ethane was the rate-determining step. It is likely that the support acidity can either promote the vanadium oxide reduction activity or increase the equilibrium constant of ethane adsorption on the catalyst.
Oxidative dehydrogenation of propane over vanadium oxide based catalysts
Catalysis Today, 2000
The oxidative dehydrogenation of propane was investigated using vanadia type catalysts supported on Al 2 O 3 , TiO 2 , ZrO 2 and MgO. The promotion of V 2 O 5 /Al 2 O 3 catalyst with alkali metals (Li, Na, K) was also attempted. Evaluation of temperature programmed reduction patterns showed that the reducibility of V species is affected by the support acid-base character. The catalytic activity is favored by the V reducibility of the catalyst as it was confirmed from runs conducted at 450-550 • C. V 2 O 5 /TiO 2 catalyst exhibits the highest activity in oxydehydrogenation of propane. The support's nature also affects the selectivity to propene; V 2 O 5 supported on Al 2 O 3 catalyst exhibits the highest selectivity. Reaction studies showed that addition of alkali metals decreases the catalytic activity in the order non-doped>Li>Na>K. Propene selectivity significantly increases in the presence of doped catalysts.
Vanadium Nitride Catalysts: Synthesis and Evaluation forn-Butane Dehydrogenation
Journal of Catalysis, 1999
A series of mesoporous vanadium nitrides with surface areas up to 60 m 2 /g was prepared via the temperature-programmed reaction of V 2 O 5 with NH 3 , and evaluated for the dehydrogenation of n-butane. Thermogravimetric analysis coupled with X-ray diffraction and scanning electron microscopy indicated that the solid-state reaction proceeded by the sequential reduction of V 2 O 5 (V 2 O 5 → V 4 O 9 → VO 2 → V 2 O 3 → VO 0.9) and concluded with the topotactic substitution of nitrogen for oxygen in VO 0.9. An experimental design was performed to determine effects of the heating rates and space velocities on the VN microstructures. The heating rates had minor effects on the surface areas and pore size distributions; however, increasing the space velocity significantly increased the surface area and decreased the average pore size. We believe the effect of the space velocity was due to inhibition of the gas-solid reactions by H 2 O and/or N 2. Temperature-programmed reduction and pretreatment studies indicated that the passivated VN could be activated by reduction in H 2 at 500 • C for 3 h. Lower reduction temperatures or times resulted in suppressed surface areas and O 2 chemisorptive uptakes. Oxygen chemisorption on the fully reduced vanadium nitrides scaled linearly with the surface area. The corresponding O/V surface ratio was 0.56 suggesting that each oxygen atom chemisorbed to approximately two surface vanadium atoms. The vanadium nitrides were highly active butane dehydrogenation catalysts with selectivities in excess of 98%. Volumetric reaction rates for the highest surface area VN were superior to those for a Pt-Sn/Al 2 O 3 catalyst. The average turnover frequency for the VN catalysts was 4.3 × 10 −4 s −1 at 450 • C assuming that oxygen chemisorbed atomically onto the active sites. The corresponding turnover frequency for the Pt-Sn/Al 2 O 3 catalyst was 6.3 × 10 −2 s −1 suggesting that while selectivities for the VN and Pt-based catalysts were similar, VN had a lower intrinsic activity.